1. Field of the Invention
The present invention relates to a technique for the Time Frequency Multiplexing (TFM) of three TV signals and, more particularly, to a technique for simultaneously transmitting three fields of a television picture signal by a Time Frequency Multiplexed signal where one field is transmitted in its original analog form in the baseband below 4.2. MHz, and the other two fields are transmitted as differential signals which are Quadrature Amplitude Modulated (QAM) onto a carrier above the baseband.
2. Description of the Prior Art
Time-Frequency-Multiplexing was originally proposed in the article "Time-Frequency Multiplexing (TFM) of Two NTSC Color TV Signals--Simulation Results" by B. G. Haskell in BSTJ, Vol. 60, No. 5, May-June 1981, at pages 643-660, for transmitting two broadcast quality color television signals on terrestrial microwave radio facilities. With TFM as then defined, two successive lines, or fields, of one picture would be frequency multiplexed so that they could be sent in one line, or field, period. During the next time interval, two successive lines, or fields, from the other picture would be transmitted, thus avoiding any crosstalk between pictures. In order to reduce the bandwidth required for the TFM signal, one of the two simultaneously transmitted lines, or fields, would be sent as a bandlimited differential signal, modulated onto a vestigial sideband (VSB) carrier frequency situated above the baseband video. The resulting TFM signal could then be transmitted through the microwave radio facility via narrow band FM.
The problem remaining in the prior art is to provide better use of the frequency spectrum by permitting the simultaneous transmission of more than two TV signals.